The present invention generally relates to a method for forming aluminum bumps and more particularly, relates to a method for forming aluminum bumps by a method that requires substantially reduced number of processing steps which includes sputtering and chemical mechanical polishing.
In the fabrication of semiconductor devices, the ever increasing device density and the decreasing device dimensions demand more stringent requirements in the packaging or interconnecting techniques for such devices. In recent years, a flip chip attachment method has been widely used in the packaging of semiconductor chips. In the flip chip attachment method, instead of attaching a semiconductor die to a lead frame in a package, an array of bumps is first formed on the surface of the die. The formation of the bumps may be carried out by a variety of methods depending on the electrically conductive material that is used to form the bumps. For instance, evaporation, electrodeposition, stencil printing, screen printing have all been used to form electrically conductive bumps on flip chips.
The more frequently utilized bump fabrication techniques are a metal deposition process and a plating process. To carry out either of the processes, a series of barrier and seed layers of metal are first deposited on the surface of the semiconductor wafer. These layers are later removed by a wet etching process everywhere except under the die pads and the layers are used to improve adhesion of subsequent layers and to form a barrier to stop metal diffusion from the bump material to the underlying die pad. In a typical bump forming process, a layer of a photoresist material is then deposited over the surface of the semiconductor wafer. A photo mask is then used to pattern the locations over each of the die pads that a bump is to be formed. An etching process, such as plasma etching is used to expose the die pads, while the openings in the photoresist layer determines the shape and height of the bump to be formed.
The electrically conductive bump, which is typically formed of gold or aluminum, can be electroplated or sputtered over the die pad and the barrier and seed layers. Once the plating or sputtering step is completed, a series of wet etching steps is used to remove the photoresist layer and the various barrier and seed layers that cover the remainder area of the wafer while the bumps protect the underlying material from being etched. While gold is the most commonly used material, other electrically conductive materials such as copper, tin-lead and aluminum as well as layered composites of these materials can also be utilized.
A conventional method for forming gold bump is illustrated in FIGS. 1Axcx9c1I. As shown in FIG. 1A, an input/output (I/O) pad 12 formed on a semiconductor substrate 14 is first provided for a semiconductor structure 10. On top of the I/O pad 12, is then deposited a passivation layer 16 of an insulating material. The passivation layer 16 is formed by a photolithographic method using a mask (not shown) to provide an opening 18 for the I/O pad 12. In the next step of the process, as shown in FIG. 1B, a diffusion barrier layer 20 of TiW is conformally deposited into the pad opening 18. On top of the TiW barrier layer 20, is then deposited a gold seed layer 22, as shown in FIG. 1C. Both the TiW barrier layer and the Au seed layer may be suitably deposited by using a sputtering technique or an electroplating technique. On top of the semiconductor structure 10, is then coated, most likely by a spin coating technique, a thick photoresist layer 24.
In the next step of the process, as shown in FIG. 1E, the photoresist layer 24 is patterned by a mask (not shown) and an opening 26 is formed by a dry etching method such as plasma etching. The opening 26 is then filled, by an electroplating process of Au, as shown in FIG. 1F. The photoresist layer 24 is then stripped by a dry etching method leaving the Au bump 28 exposed on the semiconductor 10. In the next two steps of the process, as shown in FIGS. 1H and 1I, the gold seed layer 22 is etched away by a wet etch method and then, the TiW barrier layer 20 is etched away by a wet etch method exposing only the gold bump 28 above the passivation layer 16.
The conventional gold bump forming process requires numerous photolithographic steps, numerous deposition steps and various dry etching and wet etching steps. It is a time consuming and laborious process which severely impacts the yield of the semiconductor device.
It is therefore an object of the present invention to provide a method for forming aluminum bumps that does not have drawbacks or shortcomings of the conventional bump forming techniques.
It is another object of the present invention to provide a method for forming aluminum bumps by a substantially simplified process that only requires five process steps.
It is a further object of the present invention to provide a method for forming aluminum bumps by sputtering aluminum into a plurality of openings for the bumps.
It is still another object of the present invention to provide a method for forming aluminum bumps by first sputtering aluminum into a plurality of openings and then chemical mechanical polishing to remove excess aluminum from the openings.
It is yet another further object of the present invention to provide a method for forming aluminum bumps that only requires a single photolithographic patterning process.
It is yet another object of the present invention to provide a method for forming aluminum bumps by first sputtering aluminum into a plurality of openings and then chemical mechanical polishing to remove excess aluminum from the openings.
It is yet another further object of the present invention to provide a method for forming aluminum bumps that only requires a single photolithographic patterning process, an aluminum sputtering process, a CMP process and a wet etch process.
In accordance with the present invention, a method for forming aluminum bumps by sputtering and chemical mechanical polishing is provided.
In a preferred embodiment, a method for forming aluminum bumps by sputtering and chemical mechanical polishing (CMP) can be carried out by the operating steps of providing a pre-process electronic substrate with a plurality of input/output (I/O) pads formed on a top surface; depositing an insulating material layer on top of the plurality of I/O pads to a thickness that is essentially the thickness of the aluminum bumps to be formed; photolithographically forming a plurality of openings with one on each of the plurality of I/O pads; sputter depositing a metal comprising Al filling the plurality of openings and covering a top surface of the insulating material layer; chemical mechanical polishing the electronic substrate until a plurality of Al bumps are formed with a top surfaces of the bump flush with the top surface of the insulating material layer; and removing at least partially a thickness of the insulating material layer by a wet etch process.
The method for forming aluminum bumps by sputtering and chemical mechanical polishing may further include the step of forming the plurality of I/O pads in a metal that includes Al, or the step of depositing the insulating material layer to a thickness of at least 5 xcexcm, or the step of depositing the insulating material layer from the group consisting of silicon oxide, spin-on-glass and polyimide. The method may further include the step of depositing the insulating material layer by at least two layers of different materials, or the step of depositing the insulating material layer by a first layer of Si3N4 or SiO2 and a second layer of polyimide on top of the first layer. The method may further include the step of depositing the insulating material layer by at least two layers of different materials to a total thickness of at least 5 xcexcm, or to a total thickness of between about 5 xcexcm and about 10 xcexcm. The method may further include the step of sputter depositing a metal that consists of Al and Cu, or a material that consists of Al and less than 3 wt. % Cu. The method may further include the step of conducting the wet etch process incorporating buffered oxide etch (BOE).
The present invention is further directed to a method for forming aluminum bumps on a semiconductor structure that includes the steps of providing a preprocessed semiconductor structure with a plurality of I/O pads on top; printing a layer of polyimide-containing material that has a thickness of at least 5 xcexcm on top of the structure forming a plurality of openings on each of the pluralities of I/O pads exposed; filling the plurality of openings with a metal that includes Al; removing excess metal from areas other than the plurality of opening; and removing at least partially the layer of polyimide-containing material by a wet etch process.
The method for forming aluminum bumps on a semiconductor structure may further include the step of forming the plurality of I/O pads in a metal that includes Al, or the step of printing the layer of polyimide-containing material by a screen printing or stencil printing technique, or the step of printing the layer of polyimide-containing material to a thickness between about 5 xcexcm and about 10 xcexcm. The method may further include the step of filling the plurality of openings with a metal that includes Al and Cu. The method may further include the step of removing excess metal until a surface of the metal in the plurality of openings is flush with a top surface of the layer of polyimide-containing material. The method may further include the step of removing at least partially the layer of polyimide-containing material by an etchant that includes HF and NH4F. The method may further include the step of removing at least xc2xd of a total thickness of the layer of polyimide-containing material to facilitate bonding to the aluminum bumps formed in the plurality of openings, or the step of removing completely the layer of polyimide-containing material to facilitate bonding.